HENDERSON, NV - NOVEMBER 01: Democratic presidential nominee U.S. Sen. Barack Obama (D-IL) gives the thumbs up as he arrives at a campaign rally at Coronado High School November 1, 2008 in Henderson, Nevada. Obama continues to campaign against Republican presidential nominee Sen. John McCain (R-AZ) as Election Day draws near. (Photo by Ethan Miller/Getty Images)

Photo: Ethan Miller, Getty Images

HENDERSON, NV - NOVEMBER 01: Democratic presidential nominee U.S....

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PERKASIE, PA - NOVEMBER 01: Republican presidential nominee Sen. John McCain (R-AZ) addresses a campaign rally at Pennridge Airport November 1, 2008 in Perkasie, Pennsylvania. With less than a week before the U.S. presidential election, McCain is polling double digits behind his opponent, Democratic presidential nominee Sen. Barack Obama (D-IL) in Pennsylvania, which has not voted Republican in a presidential race since 1988. (Photo by Chip Somodevilla/Getty Images)

Photo: Chip Somodevilla, Getty Images

PERKASIE, PA - NOVEMBER 01: Republican presidential nominee Sen....

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President Kennedy walks toward the rostrum in the auditorium of the new state department building in Washington, Jan. 25, 1961, to conduct his first news conference as President. Pierre Salinger, White House press secretary, is at left. Reporters stand at right. (AP Photo) President Kennedy walks toward the rostrum in the auditorium of the new state department building in Washington, Jan. 25, 1961, to conduct his first news conference as President. Pierre Salinger, White House press secretary, is at left. Reporters stand at right. (AP Photo)

Photo: AP

President Kennedy walks toward the rostrum in the auditorium of the...

Democratic presidential nominee Barack Obama often tells voters that the 2008 election is "a defining moment in history." Candidates usually think their particular race is historic, but in this case, it actually is.

Voters themselves feel they are standing at a watershed moment, whether Democrat, Republican or the dwindling conflicted few still undecided two days before the end of a two-year campaign. Half say they are scared of what might happen should the wrong man win.

The candidates themselves embody the moment: The 47-year-old Obama, on the brink of making history as the first African American president, urges voters to take the leap into the future. The 72-year-old Republican nominee, John McCain, warns that his rival is dangerously inexperienced and urges voters to stick with his record in times of peril.

Perilous times they are. The next president faces two wars, a once-in-a-century financial crisis, a warming planet, a deficit that could approach $1 trillion, unprecedented U.S. reliance on foreign oil, a teetering health care system and a battered U.S. image abroad.

Whoever prevails, three giant forces are shifting in this election: race, an enduring American preoccupation; generation, as Baby Boomers who came of age in the 1960s cede ground to the Millennials born after 1980; and ideology, with three decades of Republican dominance showing evident signs of exhaustion.

Political rarity

In purely political terms, 2008 is a rarity. Not since Republican Dwight Eisenhower faced Democrat Adlai Stevenson in 1952 has neither candidate been part of an incumbent ticket. McCain's running mate, Alaska Gov. Sarah Palin, is the first female Republican vice presidential nominee and, with Obama, ends a 219-year tradition of two white males heading the major parties.

Obama would be not only the first black U.S. president, but "so far as I know, this would be the first time a white-majority country has ever elected a nonwhite head of state," said Boston University historian Bruce Schulman. It would be the equivalent, he said, of a Jamaican-descended prime minister of Britain, an Algerian-descended president of France, or a Turkish German chancellor. "That's pretty astonishing in world historical terms."

Politics is always evolving, but there are moments in history when bigger changes occur: the 1932 election of President Franklin D. Roosevelt that ushered in the New Deal and a long Democratic reign; the political upheavals of the 1960s that began with Democratic President John F. Kennedy, saw the civil rights revolution, the Great Society and the Vietnam War under Democrat Lyndon B. Johnson before ending with the resignation of Republican President Richard Nixon; and the GOP's rebirth with the 1980 election of President Ronald Reagan.

Each reflected generational change. Stanford University political scientist Morris Fiorina, who is writing a book on the politics of generational shifts, likes to use the Rev. Jesse Jackson's gaffe during this year's campaign to illustrate.

Thinking his microphone was off during a break in a Fox News show last July, Jackson whispered to a co-panelist that Obama "talks down to black people" adding in crude terms that he would like personally to mutilate the Democratic nominee. Talk afterward was "about the aging lion versus the young cub, and Jackson being pushed aside," Fiorina said. "That's really going on everywhere."

Clinton, her husband, President Bill Clinton, and his successor, President Bush, all were products of the Vietnam era and the cultural conflicts of the 1960s. If Obama were to win and serve two terms, said Schulman, "It's hard to imagine that we would have a president again who was of that '60s generation."

The Millennial generation rivals the Baby Boom in size and, now, influence. More than a third of the voting age population was born after Watergate. Today's college freshmen, born after Reagan's presidency, have no memory of it. "We carry these things with us all our lives," Fiorina said, "and then the younger generations coming up don't have the same experiences and attitudes."

Changing attitudes

Take the strong link between age and views on gay rights or abortion. Young people take both for granted. "For every 100 people over age 70 who die and are replaced by 100 people between 18 and 24, you get more liberal social attitudes," Fiorina said.

Older generations grew up with big government programs of the New Deal and Great Society, and when those grew creaky, were open to something new, Fiorina said. "The current generation has grown up with a much more laissez-faire approach to government, and now there's a mess, so they say, OK, let's try something else."

Yet for partisans, it is the threat or hope of ideological change that matters most. The mood among conservatives has grown darker each day. Not only McCain but much of the conservative intellectual elite warn of an impending turn to European-style socialism at home and appeasement abroad, especially if Democrats seize a monopoly in Washington.

Historians call the fears exaggerated, a reflection of the country's 30-year rightward shift. On many issues, Obama is to the right of Nixon, the Republican who proposed a guaranteed income for all Americans, supported affirmative action, imposed wage and price controls, and established much of today's environmental regulation.

"A conservative in 1968 was far more liberal than a liberal is in 2008," said Schulman.

Crushing budget deficits will hamstring ambitious policy changes. Despite the likelihood of powerful Democratic majorities in Congress - they could resemble the 1964 Democratic landslide under Johnson - the deficit will constrain any Obama-style Great Society or McCain tax cuts.

Pressing economic demands will land on the next president's desk. "He didn't have any discretion over them, they're just there," said George Edwards, a presidential scholar at Texas A&M University. "That's going to be a constraint on him. The world has a way of doing that to presidents."

If McCain wins, a Democratic Congress would force him leftward as it did Nixon, reinforcing McCain's own ambivalence toward his party. Some predict he would be a caretaker president.

"McCain could not govern on economic or other issues from the right," said Christopher Malone, a political scientist at Pace University. "He's going to look at 58, 59 Democratic senators. He has no option but to govern from the center and even left of center."

McCain could emulate Eisenhower, who lost control of Congress in his first term, and only modulated Democratic policies.

Early success needed

An Obama victory offers two potential paths: a major political realignment, following Roosevelt in 1932 and Reagan in 1980. This would require that he rack up successes in his first two years, a honeymoon when presidential power is at its peak.

If he does, and realignment is under way, he could avoid the catastrophic losses that Clinton suffered after his 1993 health care plan crashed under a Democratic Congress, replaced in 1994 by a Republican one.

Another model is 1964, with a big Democratic win followed by a collapse four years later, or 1976, when Democrat Jimmy Carter ran a flawless campaign but proved a weak leader unable to control his party or rally the public.

"You could have a scenario where Obama is under a lot of pressure from his left and yet can't do big economic things because of the difficult situation we're in, where he can't disengage from Iraq and Afghanistan as quickly as the base would like," Fiorina said. Obama could then come under attack from his left, face congressional losses in 2010 and by 2012 a challenge from his own party, perhaps Hillary Rodham Clinton.

Whatever happens, many sense that the page is turning.

"This is one of those very rare, once-in-a-generation transforming elections in America that marks the end of one political era and the beginning of another," said Allan Lichtman, a political historian at American University. "That will certainly be true if Obama is elected, because he's going to come in with overwhelming control of the government. It's even true if McCain gets elected, because he's not going to be able to turn back the clock to the old conservative era."